A nuclear reactor is a source of intense heat which is generated through the exothermic fission reactions taking place inside the core. Therefore a coolant is necessary to ensure that this heat is taken away and utilized in a proper manner.
The immense amount of heat energy present in the nuclear reactor core needs to be transferred in some manner so that it is converted into electrical energy. This also helps to keep the working temperature of the core within safe limits for the materials used in the construction of the reactor. Hence a coolant plays an important role in components of nuclear power plant and serves the dual purpose of removing the heat from the reactor as well as transferring it to the electricity generation circuit either directly or indirectly depending on the type of nuclear reactor being used for the purpose.
Properties of an Ideal Coolant
There are some properties of the coolant which are necessary to ensure safety of the reactor and well as proper performance of the coolant for the intended purpose. Some of the desired properties of an ideal coolant are as follows
- A coolant should not absorb neutrons or should have a minimum neutron absorption cross section. The reason for this is obvious since this function should be left to the moderator and not the coolant.
- Since a coolant is exposed to high temperatures and well as severe levels of radiation, it is obvious that it should posses excellent resistance to both high temperatures as well as high levels of radiation.
- A coolant should be non-corrosive in nature otherwise it might tend to damage and corrode the very core which is meant to be protected by it through proper removal of heat.
- Coolants used in nuclear reactors could be either in the liquid state or in the solid state. In case the coolant is a liquid it should have a high boiling point so that it does not get evaporated due to the high heat inside the reactor. But in case it is a solid it should have a relatively low melting point due to obvious reasons.
- Since a coolant needs to circulate using a pump it should be capable of being pumped easily so that least amount of energy is spent in pumping the coolant.
It can be well imagined that the above list is quite extensive and therefore there is hardly any material which satisfies all the above criteria to the maximum possible extent. Therefore different types of coolants are used in different types of reactors depending on various factors and parameters.
Commonly Used Coolants
Since no single material qualifies as an ideal coolant, different coolants are used in different circumstances and some of the commonly used coolants are light water, heavy water, carbondioxide, helium, nitrogen, sodium, sodium-potassium mixture and so on. It can be seen that the coolants used vary from solids, liquids and gases and depending on the type of the reactor, the appropriate coolant is preferred.
It must be also kept in mind that sometimes a coolant is used to transfer heat to the working fluid in the secondary circuit through a heat exchanger while in other cases it is directly used in the turbine to rotate the blades and then fed back to the reactor after cooling in the condenser.